She left a note that indicated she would be away for several months, and was heading north.

Sifting through her last movements before she left, Clancy's family have uncovered the extent of the bullying to which she was subjected on the website Ask.fm, which encourages users to say anonymously what they really think of one another.

Cybersafety expert Susan Mclean said Ask.fm was a "hideous" site that did not have the safety features of Facebook or Twitter and whose operators were less co-operative with police.

"If you're on Ask.fm, you can expect to be abused," Mrs Mclean said.

"It's really as blunt as that.

"But young people go there with the knowledge that it's horrible and knowing they will be abused and they stay there."

The site gained a following in certain schools, though there was a groundswell of support to empower students online in northern NSW, where Ask.fm had been a problem, she said.

"People think if they live in a remote area these problems don't happen. They do. They're worse."

Ms Ellis said it appeared as though the people bullying Clancy online were known to her.

"The poor little darling could be at school thinking, 'Is that person here?'

"She said [in the note], 'I know there's no point telling you not to worry because I know you will, I know there's no point telling you not to try and find me, because I know you will' but she thinks she just needs a couple of months. She thinks this is less selfish than suicide."

Ask.fm's rules of conduct prohibit a post which is "mean, bullying someone or is intended to harass, scare or upset anyone".

Clancy is 165 to 170cm and thin, with blue eyes, dyed red hair and a piercing in her right nostril.

Police have asked anybody with information on her disappearance to contact CrimeStoppers on 1800 333 000.